Calculation of important design parameters for grounding systems in substations

An important aspect o f substation grounding system design is the calculation of the ground resistance offered by the grounding grid, and the values o f the mesh and step voltages on the surface. The values o f these parameters have to be kept within certain established Emits, keeping in view the safety o f the personnel present within and without, the substation area. Simple expressions for calculation of these parameters for square and rectangular grids are already available. Computer programs for designing substation grids, regardless o f their shape, are available, but tend to be tedious in usage when the grids are not symmetrical, since a large amount o f data has to be input to describe the grid configuration. Thus simplified expressions are preferred. Another important factor in grounding grid design, is the footing resistance, which depends on the resistivity and thickness o f the surface rock layer. The footing resistance, at present; can be determined by use of an infinite series, which is not very easy to evaluate. There is hence need for a simplified finite expression for calculating the footing resistance. The value of footing resistance also affects the safe values of the mesh and step voltages. U two substation grounding grids are intertied by a bare conductor, when a fault occurs at one substation, it is important to determine, by a simple analytical procedure, the effect of this fault current on the other grounding grid, and the response o f the system as a whole, to the fault current. The objectives o f this thesis may be summarized as follows: (i) To develop a new set of expressions, which may be used for calculation of the ground resistance, and the mesh and step voltages, for all practical shapes o f grounding grids, once information regarding the grid configuration and soil characteristics is available. (E) To develop a finite expression, for calculating the footing resistance in a substation; (Ei) To design amethod to calculate the total groundresistance o f a system of two substation grounding grids intertied by a bare conductor, as seen from the station where the fault occurs, and (lie ground potential rise at both the substations. The development o f the expressions for the calculation of the ground resistance, mesh and step voltages, footing resistance, and for evaluating the performance o f intertied grids, is described in this thesis. Through a comparison with existing expressions for the calculation of these parameters, the considerable reduction in error through the use o f the newly developed expressions is demonstrated. The scope for future work in this area is also discussed briefly.